Nickel hydroxide coated with cobaltous hydroxide

ABSTRACT

A nickel hydroxide for rechargeable batteries which is provided with a cobalt hydroxide coating which is stable to oxidation is described. The stability to oxidation is achieved by covering the surface of the cobalt hydroxide layer with anions of weak inorganic oxygen acids.

This application is the National Stage Application of PCT/EP00/01667,which claims a priority from German Application 199 10 461.1, filed Mar.10, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to a nickel hydroxide which is stable tooxidation and coated with cobalt hydroxide, and to a process for thepreparation thereof, in particular for use as a positive active mass inrechargeable alkaline batteries.

Although nickel hydroxide is outstandingly suitable for storage ofelectrical energy because of its storage capacity of 1 to (with acorresponding defect structure) a theoretical maximum of 1.67 electronsper Ni, it has a number of undesirable properties, such as lowelectrical conductivity, low cycle stability, low charging capacity athigh temperatures and the tendency to swell because of the spontaneousformation of different crystal phases with different lattice spacings.

In a very early stage of the development of nickel hydroxide batteriesit was already recognized that the properties of the batteries can beimproved by using nickel hydroxide coated with cobalt(II) hydroxide (seeU.S. Pat. No. 3,066,178). Nevertheless, the industrial use of nickelhydroxide coated with cobalt hydroxide has not since become acceptedbecause of the sensitivity of cobalt(II) hydroxide to oxidation. Rather,the route taken in the production of the battery is the use ofnon-coated nickel hydroxide with the addition of Co metal powder orcobalt compounds, such as Co(II)O or Co(OH)₂, where a cobalt hydroxidecoating crosslinked between the nickel hydroxide particles forms on thenickel hydroxide via intermediate cobalt(II) hydroxo complexes duringmore prolonged standing (typically 1 to 3 days), the cobalt compounddissolving in the electrolyte, this coating then being converted intothe actually conductive but electrochemically inactive cobaltoxyhydroxide network during the first electrical charging of thebattery. A comprehensive description of the mechanisms which areimportant here is to be found in the paper by Oshitani at the 3rdSymposium for Sectional New-Battery Study Group in Battery TechnologyCommittee of the Electrochemical Society, Dec. 11, 1986, entitled“Development of high-capacity nickel-cadmium battery using sinteredmetal fiber as substrate”. The models described there in respect ofcobalt compounds also apply to other substrates and cathode materials.

When nickel hydroxides which are coated according to the prior art witha cobalt(II) hydroxide layer which is not stable to oxidation byatmospheric oxygen are used, in the course from production via storageto the actual use in the battery a passivating cobalt(II)-containinglayer forms on the surface of the coated nickel hydroxide particles,this not only impeding the solubility of the cobalt species to give thecobalt hydroxo complexes to be formed intermediately (reduction in thecontact surface due to inadequate fusion), but additionally having apoor electrical conductivity. This then necessarily means that largeparts of the active nickel mass are not accessible electrically and asmore or less dead material can no longer contribute towards the capacityof the battery.

It has also already been proposed (Japanese Patent 25 89 123) togenerate the conductive electrochemically inactive cobalt oxyhydroxidelayer by precipitating a cobalt hydroxide layer on to the nickelhydroxide particles and subsequently oxidizing the layer in alkalinesolution by means of oxygen at a higher temperature. Such nickelhydroxide coated with cobalt oxyhydroxide is indeed stable to oxidation;however, a disadvantage is the fact that although the individualparticles in its outer shell have a good conductive layer, the formationof a three-dimensional conductive network between the individualparticles (increasing the contact surface by “fusion”) can be achievedonly by further addition of cobalt compounds which are adequatelysoluble in the alkaline electrolyte. If this addition is omitted, onlyloose contact points exist between the individual particles and thetotal resistance of the electrode is increased because of the transitionresistance which occurs between the individual particles.

On the other hand, a cobalt(II) hydroxide coating remains soluble in thealkaline electrolyte to the extent that crosslinking takes place bydynamic dissolving and addition processes, so that conductivity bridgesare generated between the particles after conversion to cobaltoxyhydroxide during the initialization charging cycle. The coatedindividual particles are “fused” in an electrically conductive manner atthe contact points of the Co(OH)₂ or CoOOH coating. A condition of this,however, is the provision of a nickel hydroxide with a cobalt(II)hydroxide coating which is stable to oxidation.

Accordingly, it has also already been proposed to treat cobalt(II)hydroxide (as a conductive additive to nickel hydroxide) or nickelhydroxide coated with cobalt(II) hydroxide with antioxidants, such asD-glucose (EP-A 744 781) or higher carboxylic acids, their esters,aldehydes, phenols or vitamins (EP-A 771 041). A disadvantage here isthat the protection against oxidation is achieved only indirectly, sincethe antioxidants only become active with respect to the oxidizedcobalt(II) form in the sense of a reduction. A further disadvantage isthat the antioxidant is consumed in the course of time, that is to saythe oxidation protection is limited with respect to time. There isfurthermore the risk that undesirable degradation products of theantioxidant are entrained into the battery.

DESCRIPTION

It has now been found that the cobalt hydroxide layer can be formed in aform which is stable to oxidation if the coated nickel hydroxide istreated with weak inorganic oxygen acids or alkali metal salts thereof.In this procedure, the surface of the coated particles is covered withanions of the acids. Suitable anions are one or more anions from thegroup consisting of aluminate, borate, carbonate, chromate, manganate,molybdate, niobate, phosphate, silicate, tantalate, titanate, vanadateand tungstate, or oxalates.

Borate, phosphate, carbonate and/or silicate are preferred. Carbonate isparticularly preferred.

“Stable to oxidization” in the context of the invention means that thedegree of oxidation of the cobalt hydroxide does not change duringstorage in air and at ambient temperature (up to 40° C.). Li, Na, Kand/or pseudo-alkali metals, such as ammonium, are suitable as thealkali metal. The treatment is preferably carried out in an aqueoussolution of the alkali metal salts, in particular of the sodium salt.

A less than monomolecular covering of the Co(OH)₂ surface is sufficientto render the nickel hydroxide provided with the Co(OH)₂ coating stableto oxidation.

It has been found that such a surface covering does not impair thesolubility of the cobalt(II) hydroxide coating in the alkalineelectrolyte of the battery, so that the effect of the formation of athree-dimensional network of high conductivity, called “fusion” above,can also take place unimpeded without the addition of a furtherconductive additive. As a result, it is possible to keep the totalamount of cobalt employed to a minimum.

The invention provides a nickel hydroxide which is provided with acobalt hydroxide layer which is stable to oxidation and the pastel greencolour of which is retained during storage in the atmosphere for aperiod of at least 4 weeks; preferably for a period of 6 months.

The invention also provides a nickel hydroxide provided with a cobalthydroxide layer which is stable to oxidation, the content of cobalt inthe 3-valent oxidation level increasing by less than 0.5%, based on thetotal content of cobalt, even after storage in air (ambient temperature10 to 35° C.) for at least 6 months.

The oxidation level of the cobalt here is preferably determined byiodometric titration in a manner known per se.

The invention furthermore provides a nickel hydroxide which is providedwith a cobalt hydroxide coating and has on its surface an at mostmonomolecular layer of anions of weak inorganic oxygen acids. Theconcentration of the anion of the weak acid is preferably 5 to 20μmol/m² of surface coated with cobalt hydroxide. A surface covering of10 to 18 μmol/m² is especially preferred.

The superficial anion concentration is preferably 10 to 50 mmol per molof cobalt(II) hydroxide, based on the amount of the cobalt hydroxidecoating.

The nickel hydroxide powder according to the invention is preferablyfree from antioxidants or organic degradation products thereof.

FIG. 1 explains the invention by comparison with the prior art.

The circles A represent diagrams of a nickel hydroxide particle, thegrey structure B represents a Co(OH)₂ coating, the broken circle Crepresents a partial oxidation of the Co(OH)₂ coating and the darkstructure D represents a CoOOH coating.

FIG. 1a) explains the prior art, according to which a nickel hydroxide(I) coated with Co(OH)₂ which is not stable to oxidation is partlyoxidized (II) on the surface by contact with ambient air. As a result,the hydroxo complex formation in the electrolyte (III) is impeded. Evenafter forming, only a partial conductivity network can thereby form(IV).

FIG. 1b) explains the generation of a CoOOH coating by conversion of theCo(OH)₂ coating by alkaline oxidation at elevated temperature (I). Theconductivity of the active mass in the battery is determined by loosepoint contacts (IV).

FIG. 1c) shows the nickel hydroxide (I, II) which is provided accordingto the invention with a Co(OH)₂ coating which is stable to oxidation,and which remains soluble (III) in the electrolyte via hydroxo complexesand therefore forms a Co(OOH) network on forming (first charging anddischarging cycles).

The nickel hydroxide to be employed according to the invention can beprepared in any desired manner.

Known processes for the preparation of nickel hydroxide are chemicalprecipitation from aqueous nickel salt solutions by means of alkalimetal hydroxide solutions, electrolytic dissolving of nickel anodes inan aqueous salt-containing electrolyte, oxidation of nickel metal powderunder pressure, dissolving of nickel powder as a complex in an ammoniasolution and subsequent precipitation by distillation, and by oxidizinghydrolysis and subsequent reduction of alkali metal nickelates.Processes of chemical precipitation or of electrolytic dissolving ofnickel anodes are preferred. A spherical nickel hydroxide which isprepared by one of the known processes of the prior art is preferablyemployed.

The nickel hydroxide base particles prepared by one of the knownprocesses are first coated with cobalt(II) hydroxide in a first step inaqueous suspension with the addition of cobalt(II) salts and alkalimetal hydroxide solution and/or ammonia under suitable conditions.Preferred conditions for achieving a uniform cobalt(II) hydroxidecoating are: continuous, semi-batch or batch process procedure,residence time 0.2 to 12 h; temperature 0 to 120° C., preferably 30 to60° C., and particularly preferably 30 to 40° C.; pressure 0.1 to 2.5bar, preferably 0.5 to 1.2 bar; pH at 25° C. 8.5 to 13, preferably 9.5to 11.5, and particularly preferably 10.2 to 10.8; solids concentration10 to 700 g/l, preferably 100 to 400 g/l; NH₃ content 0 to 15 g/l,preferably 0 to 10 g/l, particularly preferably 2 to 5 g/l; alkalimetal/cobalt ratio 75 to 150%, preferably 80 to 95% of thestoichiometry; optionally filtration, washing with water and/or dilutealkali metal hydroxide solution, preferably sodium hydroxide solutionwith a pH of 11 to 12. The coating with cobalt(II) hydroxide can also becarried out without an inert gas or the addition of antioxidants.

To stabilize the cobalt(II) hydroxide coating by rendering the surfaceinert, the original precipitation suspension of the coating process or,preferably, the nickel hydroxide which has been coated with cobalt(II)hydroxide and has already been separated off from the mother liquor andresuspended in water is then treated in a second step with the weakacids listed above or aqueous alkali metal salts or alkali metalhydrogen salts thereof. The stabilizing treatment can also be carriedout directly after filtration of the mother liquor of the originalprecipitation suspension of the coating process, by treatment of theunwashed or washed filter cake. The temperature range of the stabilizingtreatment is 0 to 100° C., preferably 20 to 60° C., and particularlypreferably 40 to 50° C. The duration of the treatment can be 0.2 to 12hours.

It has proved to be particularly advantageous to carry out thestabilizing treatment by carbonation of the surface of the nickelhydroxide coated with cobalt(II) hydroxide. The carbonation here ispreferably carried out by addition of alkali metal carbonate and/oralkali metal bicarbonate solutions at concentrations from 0.01 mol/l upto the solubility maximum, preferably 0.03 to 1 mol/l, the amount ofalkali metal carbonate and/or alkali metal bicarbonate solution being0.1 to 10 times the weight, based on the solids content of thesuspension which comprises the nickel hydroxide coated with cobalt(II)hydroxide. In the context of the invention, the carbonation then alsoproceeds if the carbonation takes place by addition of carbon dioxide,e.g. introduction as a gas.

A particularly preferred embodiment of the invention comprises treatingthe still moist filter cake from the cobalt hydroxide coating or thesubsequent washing in carbon dioxide or carbon dioxide-containing airunder CO₂ partial pressures of 0.01 to 2 bar, preferably 0.01 to 1 bar.The treatment can be carried out simultaneously with the drying of thepowders, for example in continuously operated spray dryers or spin flashdryers.

The nickel hydroxide coated with a stabilized cobalt(II) hydroxide layerafter the stabilizing treatment has been concluded can optionally alsoadditionally be washed before the final drying step. The drying itselfdoes not have to be carried out with exclusion of air (vacuum or inertgas), but drying can be carried out in air to save costs. All theconventional types of dryer can be used as drying units.

Although pulverulent nickel hydroxide is preferred according to theinvention, the invention is not limited to pulverulent nickel hydroxide.According to the invention, it is possible in an analogous manner firstto precipitate nickel hydroxide on to a substrate, preferably a nickelgauze, nickel nonwoven or a nickel foil, subsequently to coat this withcobalt hydroxide and then to immerse the coated substrate into thetreatment solution.

Preferred base nickel hydroxides have average particle sizes (D50 value,measured by the Mastersizer method) of 0.5 to 500 μm, particularlypreferably 2 to 30 μm. The specific surface area can advantageously be 2to 70 m²/g, measured by the BET method.

The base nickel hydroxide can furthermore comprise one or more dopingelements known per se from the group consisting of magnesium, calcium,strontium, scandium, yttrium, lanthanum, lanthanoids, titanium,zirconium, chromium, molybdenum, tungsten, manganese, iron, cobalt,copper, cadmium, zinc, boron, aluminium, gallium, indium, silicon,phosphorus, arsenic, antimony and bismuth, in amounts of 0.2 to 25 wt. %in total. Preferred nickel hydroxides are doped with 0.5 to 5 wt. % zincand 0.5 to 5 wt. % cobalt in a total amount of 3 to 8 wt. %.

The cobalt hydroxide coating can also comprise doping elements from theabovementioned group in a manner known per se.

The invention is further described in the following illustrativeexamples in which all parts and percentages are by weight unlessotherwise indicated.

The invention is illustrated in more detail by the following example:

EXAMPLE

348.5 g/h of a CoCl₂ solution (7.0% Co), 297.5 g/h of a 10% NaOHsolution, 180 ml/h of a 100 g/l NH₃ solution and 3,000 ml/h of anNi(OH)₂ suspension (150 g/l of spherical Ni(OH)₂ doped with Co and Zn,80 g/l NaCl, 3.5 g/l Na₂SO₄) are continuously metered in parallel into a17 liter stirred reactor temperature-controlled at 37° C. A pH (25° C.)of 10.5 and an NH₃ content (Kjeldahl) of approx. 3.5 g/l are establishedhere in the equilibrium state. After a first runnings time of 24 hours,the outflow from the reactor (approx. 3,750 ml/h) is collected at 24 hintervals for a further 72 hours, filtered batchwise and washed with ineach case 23 kg of temperature-controlled 0.2 g/l NaOH solution (45°C.). The residue on the filter is then treated with 46 kg 1.0 mol/lNa₂CO₃ solution (45° C.) and finally washed with 46 kg completelydemineralized water (45° C.). Drying is carried out at 50° C. in vacuo.The product yield is 11.7 kg per 24 h batch.

Table 1 shows analytical values of the non-coated base nickel hydroxide.Table 2 shows analytical values of the nickel hydroxide coated withcobalt hydroxide with oxidation-stabilizing treatment immediately afterpreparation and after storage in air for 6 months.

TABLE 1 Ni(OH)₂ base material, non-coated Ni Co_(tot.) Co(III) Zn Cl NaSO₄ C (%) (%) (%) (%) (ppm) (ppm) (ppm) (ppm) 57.17 1.44 0.5 3.65 360 3180 1,100 TD MS CS BET (g/cm³) D₅₀(μm) (A) (m²/g) 2.0 4.5 70 19.84

TABLE 2 Ni(OH)₂ base material according to the invention, coated withCo(OH)₂ and CO₃ Ni Co_(tot.) Co(III) Zn Cl Na SO₄ C (%) (%) (%) (%)(ppm) (ppm) (ppm) (ppm) 52.92 6.44 0.7 3.36 420 35 140 1,400 Co(III)after storage in air for 6 TD MS CS BET months (g/cm³) D₅₀(μm) (A)(m²/g) (%) 2.0 5.11 70 17.42 0.7 TD = tap density; MS = particle sizedetermined by the Mastersizer method; CS = crystallite size determinedby X-ray analysis; BET = specific surface area measured by the BETmethod, % values = wt. %.

The nickel hydroxide material coated according to the invention andprepared according to the example is measured, after storage in air for6 months, in the half cell test without any addition of a conductiveadditive in the electrode production and, from the 3rd (charging anddischarging) cycle, reaches 99% of the Ni utilization, based on theone-electron step.

A conventional three-electrode arrangement which comprises an Hg/HgOreference electrode, a nickel plate counter-electrode and the workingelectrode comprising the nickel hydroxide active mass is chosen here.The electrochemical measurement takes place galvanostatically, i.e. aconstant charging and discharging current is set between the workingelectrode and counter-electrode. The potential of the working electrodeagainst the potential of the reference electrode is measured. The cyclicoperation is effected by 15 hours of charging with I₁₀ (I₁₀ designatesthe charging current which delivers 100% of the theoretical chargingcapacity in 10 hours) and discharging with I₁₀ to 0 V vs. Hg/HgO. Anaqueous potassium hydroxide solution is used as the electrolyte. Toprepare the working electrode, the nickel hydroxide material accordingto the invention is prepared as a foam electrode and then measured.

Although the present invention has been described in detail withreference to certain preferred versions thereof, other variations arepossible. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of the versions contained therein.

What is claimed is:
 1. A coated nickel hydroxide having a cobalthydroxide coating, wherein the nickel hydroxide is stable to oxidation,and wherein the coating has 1 to 200 mmol of one or more anions of weakinorganic oxygen acids per mol of cobalt(II) hydroxide on a surface ofthe coating, and wherein the anions form, at most, a monomolecularlayer.
 2. The nickel hydroxide according to claim 1, wherein the anionis CO₃.
 3. The nickel hydroxide according to claim 1, wherein the nickelhydroxide is in the form of powder and wherein the nickel hydroxide hasan average particle size (D50 value, measured by the Mastersizer method)of 0.6 to 500 μm.
 4. The nickel hydroxide according to claim 1, whereinthe nickel hydroxide is a coating on a substrate.
 5. The nickelhydroxide according to claim 1, wherein the nickel hydroxide comprisesan amount of 0.2 to 25 wt. % in total of a doping element selected fromthe group consisting of Mg, Ca, Sr, Sc, Y, La, lanthanoids, Ti, Zr, Cr,Mo, W, Mn, Fe, Co, Cu, Zn, Cd, B, Al, Ga, In, Si, P, As, Sb and Bi, andcombinations thereof.
 6. The nickel hydroxide according to claim 1,wherein the nickel hydroxide has water molecules at interstitial sitesin an amount of up to 10 wt. %.
 7. The coated nickel hydroxide of claim1, wherein a doped or non-doped nickel hydroxide is coated withstabilized cobalt(II) hydroxide.
 8. An electrode material in a secondarybattery comprising the nickel hydroxide of claim 7.